Synonyms: Abrothallus exilis auct. non (Flörke) A. Massal.; Allarthonia exilis auct. non (Flörke) Sandst.; Arthonia exilis auct.; Arthonia rugulosa (Kremp.) Almq.; Catillaria exilis auct.; Lecania zinaidae Oxner; Lecidea synothea var. exilis Flörke
Distribution: N - Frl, Ven (Lazzarin 2000b, Nascimbene & al. 2005b, Thor & Nascimbene 2007), TAA (Nascimbene & al. 2007b, Nascimbene 2014), Lomb, Piem (S-F68607). C - Marc (Nimis & Tretiach 1999), Abr (Nimis & Tretiach 1999, Gheza & al. 2021), Mol (Nimis & Tretiach 1999, Caporale & al. 2008), Sar (Zedda 2002, Rizzi & al. 2011). S - Pugl (Nimis & Tretiach 1999), Bas (Nimis & Tretiach 1999), Cal (van den Boom & Khodosovtsev 2004), Si (Nimis & al. 1994, Grillo & Cristaudo 1995).
Description: Thallus crustose, inconspicuous, more or less continuous, grey-brown or olive-brown, of brownish hyphae loosely growing between the algal cells, but tightly arranged around them. Apothecia arthonioid, black, 0.15-0.4 mm across, irregular in shape but not much longer than wide (not lirelliform), with a convex disc and a poorly visible proper margin. Epithecium grey-brown, of branched hyphae in a gelatinous matrix with granules; hymenium colourless to very pale red-brown, 40-60 µm high, I+ red, K/I+ blue; paraphysoids slender, c. 1 µm thick, the terminal parts extending periclinally along the surface of the epithecium, the apical cells brown-black-capped and 3-4.5 µm wide; hypothecium colourless to pale olive brown. Asci 8-spored, subglobose, semi-fissitunicate, with a large apical dome and a distinct ocular chamber, Arthonia-type. Ascospores 1-septate and constricted at septum, with unequal cells, hyaline, ovoid, (10-)12-16 x 3.5-5(-6) µm, with a very thin gelatinous sheath. Photobiont chlorococcoid. Spot tests: K-, C-, KC-, P-. Chemistry: without lichen substances.
Note: a mainly temperate species found on base-rich and soft bark, especially of Sambucus, and on young twigs of deciduous trees in sheltered situations, with optimum in the submediterranean belt. The species is heterogeneous, and Italian material needs revision.